Process for the production of elastomer-modified thermoplastics

ABSTRACT

The present invention relates to a continuous process for the production of impact resistance-modified thermoplastics. An elastomer, in particular a particulate graft polymer with a water moisture content of 1 to 50%, preferably 10 to 40%, is incorporated into a thermoplastics melt in a large volume kneader reactor. The mixing ratio of elastomer to thermoplastics melt is 4:1 to 1:4. The energy required to melt the elastomer and to evaporate the adhering water is added by heating the walls of the apparatus and via the kneading action of the installed rotating parts.

CROSS-REFERENCE TO RELATED APPLICATION

The present application is a division of application Ser. No.09/044,410, filed Mar. 19, 1998.

The present invention relates to a continuous process for the productionof elastomer-modified thermoplastics. An elastomer, in particular, anoptionally particulate graft polymer with a water moisture content of 1to 50%, preferably 10 to 40%, is incorporated in a large volume kneaderreactor into a thermoplastics melt. The mixing ratio of elastomer tothermoplastics melt is in particular 4:1 to 1:4. The energy required tomelt the elastomer and to evaporate the adhering water is added byheating the apparatus walls and through the kneading action of theinstalled rotating parts.

Elastomer-modified thermoplastic polymers, for example rubber-filledplastics, are normally produced starting from rubber by emulsion graftpolymerisation onto a rubber base. The thermoplastic matrix can beeconomically produced in a bulk or solution process. In emulsionpolymerisation a graft polymer for example is produced having a highrubber content (typically 50 wt. %), which after being worked up iscompounded with a matrix polymer. The working up of the graft latextakes place via the following processing steps: precipitation, washing,mechanical and thermal drying.

The thermal drying of a graft latex in the solid phase requires ofcourse a very high energy input, and on account of the danger of a dustexplosion associated with the drying this procedure is carried out underinert conditions in driers. An object of the invention is accordingly toreduce considerably the energy expenditure compared to theaforementioned processes.

In addition to the commonly employed combination of powder dryingfollowed by compounding with the thermoplastic material, processes forthe impact resistance modification of thermoplastics have already beendescribed in the prior art, which are based on incorporating rubberlattices that have been mechanically only partially dewatered, directlyinto thermoplastic polymers in a screw extruder (see for example D20 37784). Modified extruder processes are described in the Europeanlaid-open specifications EP 0 534 235 A1, EP 0 665 095 A1, EP 0 735 077A1, EP 0 735 078 A1, EP 0 734 825 A1 and EP 0734 826 A1.

A particular disadvantage of these processes is the high degree ofstress to which the rubber/thermoplastics mixture is subjected onaccount of the high shear rate of up to 1000 s⁻¹ in screw extruders.

A further disadvantage is the multistage procedure involved in thelast-mentioned process, since first of all water and then the meltmixture is removed, and in a further step the residual degassing of thepolymer is carried out.

Since the energy in the extruder is mainly introduced as mechanicalenergy via the screw shafts, it is possible only to a limited extent tocontrol the energy input via the added heat and avoid thermallystressing the polymers.

The object of the invention is to provide a direct processing methodthat avoids the danger of dust explosion inherent in processes involvingpowder drying and that overcomes the various disadvantages of processingin screw extruders.

The invention provides, by means of which the aforementioned objectiveis achieved, a process for producing elastomer-modified thermoplasticsby mixing elastomer, especially rubber, that is surrounded by graftpolymerisation with a sheath of a thermoplastics material A, with athermoplastics material B, the moist elastomer, especially a graftpolymer that has been precipitated from a latex, being dewatered to 1 to50 wt. %, especially 5 to 50 wt. %, and particularly preferably 10 to 40wt. % residual moisture, and into which is mixed the thermoplasticsmaterial B present as melt, which process is characterised in that theevaporation of the process water adhering to the elastomer, especiallyto the graft polymer, the melting of the elastomer, the compounding ofthe elastomer with the melt of the thermoplastics material B, as well asthe removal of further organic volatile constituents C, are carried outsimultaneously in one process chamber.

The dewatering in the first stage is preferably carried outmechanically, for example by squeezing out the water or bycentrifugation.

In particular, the energy necessary for the melting, heating andevaporation of the polymer mixture is introduced mechanically via thekneading action of the rotors, and thermally via the surface of thehousing of the kneader reactor, the ratio of the mechanical energy andthermal energy to be added to the mixture preferably being 4:1 to 1:6,particularly preferably 2.5:1 to 1:4.

The process is preferably carried out in a large volume, partiallyfilled kneader reactor with installed rotating parts, in which thethroughput per liter of process space is not more than 5 kg/hr ofpolymer. The residence time of the miter in the process space ispreferably 2 to 20 minutes.

The kneading action of the rotors is influenced in particularindependently of their rotational speed, by controlling the degree offilling of the machine.

In a further preferred variant of the process the water adhering to theelastomer and further volatile constituents C are removed in only onepressure stage, in particular at a pressure of 10 to 2000 hPa.

The dewatering of the polymers is carried out in particular in a kneaderreactor in which the mixing and kneading actions of the installedkneader reactor parts are largely decoupled from the transportingmovement of the material being mixed.

The mixing ratio of thermoplastics material B and elastomer can in apreferred process be varied in the range from 1:4 to 4:1 withoutaltering the process parameters or the equipment configuration used inthe process.

In the mixing of elastomer and thermoplastics material B, in particularadditional pigments and/or additives are prepared and processed so thatthey can be added to the mixture before the latter is dewatered.

In a process variant pigments and/or additives are preferably added tothe polymer mixture in a mixer, in particular a static mixer, connecteddownstream of the process chamber.

Suitable kneader reactors for carrying out the process according to theinvention are those which can successfully mix tough plastic phases, forexample those disclosed in the publications EP 0 517 068 A1, EP 460 466B1, EP 0 528 210 A1 or JP-A-63232828. Twin-shaft reactors correspondingto specification EP 0 517 068 A1 are preferably used. Since themechanical stress to which the rotors are subjected and the requiredpower are in some cases considerably higher than in conventionalapplications of this type of equipment, it may be necessary withcommercially available equipment to reinforce the rotors and choose aconsiderably more powerful drive unit compared to the conventionalspecification.

The moist elastomer, especially graft polymer, is in a preferredembodiment added by means of a stuffing screw or a piston lock.Alternatively the elastomer may be added via a strainer-type orsqueezer-type screw with partial mechanical removal of the moisture.Furthermore, in the preferred embodiment the thermoplastics melt isadded via the inlet-side front plate of the kneader reactor, whichprevents the generally temperature-sensitive elastomer coming intocontact with the hot surfaces of the housing. Instead, as soon as theelastomer enters the large volume kneader reactor it is embedded in thethermoplastics melt B. Also the harmful influence of any possibleby-products on the resultant mire due to the relatively long eductresidence time in the initial section of the kneader reactor is avoided.The dewatered, degassed and compounded, elastomer-modified thermoplasticmaterial is discharged from the kneader reactor preferably via adischarge screw or gear pump at or in the vicinity of the front plateopposite the inlet. The reactor volume is optimally utilised by means ofthis arrangement. A melt screening and granulation stage according tothe prior art may be coupled to the discharge unit. The vapours areremoved through a degassing vent which is preferably arranged in thevicinity of the product discharge outlet, and can then be condensed in amanner known per se. If the degassing vent is arranged closer to thefeed point there is an increased danger that the yield will be reduceddue to escape of powder. Furthermore, in the preferred embodiment thedegassing vent is cleaned by a screw, which prevents melt passing intothe vapour channel and forming blockages. In the preferred embodimentall surfaces of the kneader reactor that come into contact with theproduct are also heated. In this way the energy input into the processchamber is maximised, which means that the processes can be operated inan economically optimum manner. The process is preferably carried out atan internal pressure of 1 hPa to 5000 hPa, especially 10 to 2000 hPa,but most preferably at normal pressure, optionally also under theaddition of inert gases. The temperature of the heated apparatus wallsis in particular 150 to 350° C. preferably 180 to 300° C., andparticularly preferably 200 to 270° C. The specific power for a reactorwith installed rotating parts is in particular 0.01 to 1 kWh per kg ofdry polymer melt, preferably 0.05 to 0.5 kWh/kg, and particularlypreferably 0.05 to 0.25 kWh/kg.

Suitable graft polymers for carrying out the process according to theinvention are graft-polymerised vinyl compounds, for example styrene,alpha-methylstyrene, acrylonitrile, methyl methacrylate and alkylacrylate or mixtures thereof, particularly preferred are methylmethacrylate as well as mixtures of styrene and acrylonitrile,alpha-methylstyrene and acrylonitrile, methyl methacrylate and styrene,methyl methacrylate and alkyl acrylate, and alpha-methylstyrene, methylmethacrylate and acrylonitrile. Suitable rubbers (graft bases) are dienehomopolymers and copolymers, for example of butadiene, isoprene,chloroprene, optionally with up to 35 wt. % of comonomers such asstyrene, acrylonitrile, methyl methacrylate, alkyl acrylate, vinylmethyl ether, or alkyl acrylate polymers (in particular of C₁-C₁₀ alkylacrylates), which optionally contain up to 20 wt. % of vinyl monomersincorporated by polymerisation, such as styrene, acrylonitrile, vinylacetate, or C₁-C₁₀-alkyl methacrylate; the acrylate rubbers areoptionally partially crosslinked by incorporating by polymerisationpolyfunctional vinyl or allyl monomers; crosslinking monomers are forexample bis-acrylates, bis-acrylamides, butadiene, vinyl acrylate,triallyl cyanurate, trialkyl isocyanurate, trisallyl citrate, orbis-carboxylic acid vinyl ester.

As elastomer there may be used in the process according to the inventionany polymer that has elastomeric properties and that can be added to anextruder. Suitable rubbers are for example nitrile rubbers and partiallysaponified nitrile rubbers. Particulate rubbers in particular are used.Especially preferred are those rubbers that have a grafted-on sheath ofanother substance, generally not the elastomeric thermoplastic materialA.

Acrylate rubbers contain the crosslinking monomers in amounts of up to amaximum of 5 wt. %. The rubbers may also have a core/jacket structure,i.e. the acrylate rubber particles contain a rubber core that isstructurally different from the surrounding acrylate rubber, or a hardthermoplastic resin core. In particular stages may be polymerized fromone or more of the monomers styrene, alkyl styrene, acrylonitrile ormethyl methacrylate. Preferred are graft polymers based onbutadiene/styrene/acrylonitrile, n-butyl acrylate/styrene/acrylonitrile,butadiene/n-butyl acrylate/styrene/acrylonitrile, n-butylacrylate/styrene/methyl methacrylate,butadiene/styrene/acrylonitrile/methyl methacrylate, andbutadiene/n-butyl acrylate/methyl methacrylate/styrene/acrylonitrile.

Together with these preferred graft polymers, styrene/acrylonitrile(SAN) copolymers, polystyrene, polymethyl methacrylate, polyvinylchloride or mixtures of these polymers are used as thermoplasticmaterials B.

In this connection SAN polymers, polymethyl methacrylate (PMMA) ormixtures of these polymers are particularly preferred as thermoplasticsmaterial B. Copolymers that are particularly preferably used areobtained from 20 to 40 wt. % of acrylonitrile and 80 to 60 wt. % ofstyrene or alpha-methyl styrene. These copolymers are known and can beprepared by free-radical polymerisation, for example in emulsion,suspension, solution or bulk The copolymers preferably have a molecularweight M_(w) of 15 000 to 200 000.

Furthermore polycarbonate, poly(butylene terephthalate),poly(oxymethylene), poly(methyl methacrylate), poly(phenylene sulphide),polysulphones, poly(ether sulphone)s and polyamides and mixtures ofthese thermoplastics may also be used as thermoplastic material B.

The graft polymer lattices generally contain 30 to 90 wt. % of rubber,and preferably 50 to 85 wt. %.

The vinyl polymer lattices may be prepared in a known manner by emulsionpolymerisation in aqueous media or emulsion graft polymerisation in thepresence of rubber lattices. In the case of rubber-free polymers themonomers are polymerised in aqueous media by a free-radical reaction inthe presence of soaps (emulsifiers) at pH values of ca. 12 to 2, inparticular 10 to 3. As initiators there may be used in particularwater-soluble radical-formers such as peroxo disulphates, peroxodiphosphates, water-soluble hydroperoxides and peroxo acids, as well asredox initiator systems. The polymerisation, which is normally carriedat 40 to 90° C., requires the presence of an ionic emulsifier,especially an anionic emulsifier, in amounts of up to 4 wt. %,preferably up to 2.5 wt. %, with respect to the monomers. Suitableemulsifiers are for example fatty acid salts, alkylsulphonic acid saltswith long-chain alkyl radicals and sulphuric acid alkyl semiesters withlong-chain alkyl radicals as well as, preferably, alkali salts ofdisproportionated abietic acid.

The vinyl polymer lattices prepared in this way generally have a polymersolids content of 10 to 70 wt. %, preferably 25 to 50 wt. %. Theproportion of non-polymerised monomers in the latex is generally 0 to 15wt. %, preferably 0 to 5 wt. %, with respect to the polymer solidscontent of the latex. The size of the vinyl polymer latex particles is50 to 1000 nm, preferably 80 to 650 nm.

The lattices are coagulated by processes well-known per se (see forexample EP 459 161 A2).

As coagulating agent there are preferably used aqueous solutions ofwater-soluble inorganic or organic acids and/or their salts, such ashydrochloric acid, sulphuric acid, phosphoric acid, boric acid, formicacid, acetic acid, propionic acid, citric acid, alkali metal andalkaline earth metal chlorides, sulphates, formates, acetates,phosphates, carbonates, aluminates, partially saponified polyvinylacetates, optionally in combination with inorganic or organic acids. 0.2to 25 wt. % of aqueous solutions are used depending on the vinyl polymerlatex mixture to be coagulated.

The expression organic volatile constituents C denotes monomers and lowmolecular weight oligomers of the employed thermoplastic polymers and/orelastomers or graft polymers (for example styrene, acrylonitrile),emulsifier constituents (for example dihydroabietic acid) or solventsthat are used in the preparation of the graft polymers and/orthermoplastics (for example ethylbenzene, methyl ethyl ketone).

In contrast with the known extrusion processes, the process according tothe invention is characterized by a gentle and optionally continuousprocessing of the starting components. In extruders, i.e. in thedescribed known processes with twin-shaft screws rotating in the samedirection, the highest shear rates occur between the screw shafts andbetween the screw shaft and the housing wall. Under conventional knownprocessing conditions this shear rate is of the order of magnitude of1000 s⁻¹, and is thus considerably higher than the mean shear ratequoted in EP 0 734 825 A1, which is governed by the less stronglysheared flow in the screw channel. However, it is the maximum stressthat is relevant in evaluating the damage to the product. During theprocessing in the kneader reactor this maximum stress is of an order ofmagnitude of 100 s⁻¹, and is thus considerably lower. A particularfeature of the process according to the invention is that for examplethe shear-sensitive graft polymer is mixed with thermoplastics meltimmediately after entry into the reactor. In the extrusion processaccording to EP 0 735 077 A1, EP 0 735 078 A1, EP 0 734 825 A1, and EP 0734 826 A1 however, the graft polymer is already subjected to a highshear rate before a lubricating action takes place as a result of theaddition of thermoplastics melt. Moreover, the thermal stress to whichthe polymer is subjected is also less since the addition of energy,which occurs in a dissipative manner via the kneading action of therotors and thermally via the heating of the walls, can be matched by afree choice of the rotor speed and the heating temperature to the energyrequired to evaporate the water. When processing in extruders thepossibility of influencing the processing by controlling the temperatureof the housing is slight since by far the greatest proportion of theenergy is added via the dissipative action of the screw shafts. Theprocess steps of water evaporation and residual degassing of the polymertake place is succession in the extruder, whereas they occursimultaneously for example in the kneader reactor. Inevitably furthermechanical energy is thereby added to the extruder after completion ofthe water evaporation step, which results in a further thermal stress onthe polymer. In the process according to the invention the evaporationof water and other volatile constituents occurs at the same time. Theprocess can thus be controlled so that the elastomer-modifiedthermoplastic material is discharged from the reactor after completionof the water evaporation step without subjecting the melt to any furtherthermal stress.

In contrast to the known processes, the process according to theinvention is characterised by a high yield. Whereas in the knownextrusion processes up to 10% of polymer is discharged together with theexpressed water in the squeezing zones and the evaporation rates in theevaporation zones are high on account of the small cross-sections of thescrew channel with the resultant danger that polymer will be entrainedwith the vapours, in the process according to the invention there ispractically no loss of product. On account of the large freecross-section of for example a kneader reactor, the evaporation rate isalways so low that no polymer is entrained.

The aforementioned advantages result in a high degree of flexibility,which is yet a further advantage of the process according to theinvention. On account of the gentle processing conditions mixturescontaining a very high proportion of elastomer can be processed withoutdamaging the product. Also, mixtures with a comparatively high watercontent can be processed without adversely affecting the yield.

Several process steps take place in succession in the extruder, moreprecisely the steps described in detail in the printed specifications EP0 735 077 A1, EP 0 735 078 A1, EP 0 734 825 A1 and EP 0 734 826 A1, ineach case the screw geometry being adapted to the process step and theproduct to be processed. In the kneader reactor the melting of theelastomer, for example the graft polymer, the compounding of the twomelts, and the evaporation and degassing of water as well as optionallyof monomers and other organic volatile constituents are carried outsimultaneously in one process chamber. It is therefore in particular notnecessary to adapt the reactor to particular product properties.Mixtures of elastomer and thermoplastic matrix in a mixing ratio of 1:4to 4:1 are of particular practical interest. The whole spectrum of thesepolymer blends can be produced in one and the same machine without anyspecial adaptation. In particular, pigments and/or additives can also beadded already during the preparation of the polymer mixture, with theresult that a further compounding can be omitted. The process isaccordingly characterised by a high degree of simplicity. The process isa one-stage process, whereas the known extrusion processes arenecessarily multistage processes. If the process is preferably carriedout under normal pressure, no special vacuum technology is required.Despite this, a low residual content of volatile constituents isachieved. All commercially available processes that use extruders toachieve the desired objective operate with additional vacuum technology.

The mean residence time of the product when carrying out the process inthe process chamber, in particular in a kneader reactor, is typically 2to 20 minutes, preferably 3 to 10 minutes. The residence time is thusconsiderably longer than in a conventional extrusion process, where thetypical residence time is less than 1 minute. It has surprisingly beenfound however that better colour values of the final polymer mixture canbe achieved with the process according to the invention than with anextrusion process.

EXAMPLES

In the following examples the following polymers are used:

Composition A)

A partially crosslinked polybutadiene latex grafted with styrene andacrylonitrile in a weight ratio of 72:28 (mean diameter of the latexparticles d₅₀=400 nm) with a polymer content of 36.5 wt. % and aresidual monomer content of 0.75 wt. %, with respect to the polymer. Thegraft polymer contains 55 wt. % of rubber.

Composition B)

A partially crosslinked polybutadiene latex grafted with styrene andacrylonitrile in a weight ratio of 72:28 (mean diameter of the latexparticles d₅₀=120 nm) with a polymer content of 36.5 wt. % and aresidual monomer content of 0.5 wt. %, with respect to the polymer. Thegraft polymer contains 50 wt. % of rubber.

The lattices are prepared according to known methods (cf. U.S. Pat. No.4,399,273). The mean particle diameter (d₅₀ value) is determined byultracentrifuge measurement (W. Scholtan, H. Lange; Kolloidz. und Z.Polymere 250 (1972) 782-796).

Graft polymer BMG:

A mixture of 50 wt. % of each of the vinyl polymer lattices A and B iscoagulated in a falling cascade at above 92° C. by adding MgSO₄ andacetic acid, and is then neutralised, filtered, washed free ofelectrolyte, and after washing is dewatered in a centrifuge or through aband press to 28 to 35 wt. % water content. This product is used for thefurther examples.

Graft polymer P50:

Graft polymer latex A is treated like graft polymer B, coagulated, andafter washing is dewatered in a centrifuge or through a band press to 28to 35 wt. % water content. This product is used for the furtherexamples.

Thermoplastic SAN M60 (L-value 60):

Styrene/acrylonitrile copolymer comprising 72 wt. % of styrene and 28wt. % of acrylonitrile, with a molecular weight (weight modulus) M_(w)of ca. 80 000 and a non-uniformity U=M_(w)/M_(N)−1=2.

Example 1

10.625 kg per hour of a styrene/acrylonitrile copolymer (SAN) with anL-value of 60 are added at a melt temperature of 230° C. to a modifiedORP12 reactor manufactured by List AG, Arisdorf (Switzerland), withrotors reinforced as regards torsional resistance. The reactor has ausable volume of 30.61. 49 kg per hour of a graft polymer BMG containing34.6% of water are also added in powder form by means of a stuffingscrew to the reactor. The rotational speed of the two rotors of thereactor is 100 and 25 revolutions per minute. The molten, compoundedproduct free of volatile constituents is extruded at a temperature of237° C. by means of a twin-shaft extrusion screw. The reactor walls andthe rotors are heated with thermal oil at a temperature of 275° C. Amechanical output of 3 kW is taken off at the rotor shafts. A heatingcapacity of about 12 kW is added via the apparatus walls. Overall, aspecific capacity of 0.47 kWh/kg is added with respect to the graftpolymer. The mean residence time of the product in the apparatus is ca.11 minutes, i.e. the apparatus is on average filled to about 27% withpolymer mixture.

GC-analysis shows a residual content of volatile constituents of 320 ppmin the product. The educts contained around 2000 ppm (from the graftpolymer) and 1000 ppm (from SAN) of volatile components. The resultantproduct is kneaded in a ratio of 40:60 with styrene/acrylonitrilecopolymers (SAN) with an L-value of 60 and extruded in the form ofplatelets for colour evaluation.

Comparison Example 1

By way of comparison, a graft polymer that has undergone a thermaldrying is kneaded with a styrene/acrylonitrile copolymer (SAN) having anL-value of 60, so as to produce the same rubber content. A comparison ofthe colour pattern plates from both processes did not reveal anydifference in colour.

The energy expenditure just for the thermal drying of the graft polymerin an open flow drier is 0.85 kWh/kg.

Example 2

70 kg per hour of a styrene/acrylonitrile copolymer (SAN) with anL-value of 60 are added at a melt temperature of 230° C. to the ORP 12reactor manufactured by List AG, Arisdorf (Switzerland). The reactor hasa usable volume of 30.61. 42.6 kg per hour of a graft polymer P50containing 29% of water are also added in powder form by means of astuffing screw to the reactor. The rotational speed of the two rotors ofthe reactor is 120 and 30 revolutions per minute. The energy added viathe rotor shafts is 7.6 kW. 4.8 kW of energy is supplied through thewall of the housing. The degree of filling of the apparatus is 36% (withrespect to the polymer mixture) and the mean residence time of themixture in the apparatus is 6.5 minutes. The molten, compounded productfree of volatile constituents is extruded at 240° C. by means of atwin-shaft extrusion screw. The reactor walls and rotors are heated withthermal oil at a temperature of 270° C.

Example 3

67.18 kg per hour of a styrene/acrylonitrile copolymer (SAN) with anL-value of 60 are added at a melt temperature of 230° C. to a CRP 12reactor with reinforced rotors, manufactured by List AG, Arisdorf(Switzerland). The reactor has a usable volume of 30.61. 44 kg per hourof a graft polymer containing 34.5% of water are likewise added in theform of powdered BMG by means of a stuffing screw to the reactor. Therotational speed of the two rotors of the reactor is 100 and 80revolutions per minute. The energy added via the rotor shafts is 15.2kW. The degree of filling of the apparatus is 31% and the mean residencetime in the apparatus is 6 minutes. The molten, compounded product freeof volatile constituents is extruded at 234° C. by means of a twin-shaftextrusion screw. The reactor walls and rotors are heated with thermaloil at a temperature of 250° C.

A GC analysis shows a residual content of volatile constituents of 60ppm in the product. The educts contained around 2000 ppm (from the graftpolymer) and 1000 ppm (from SAN) of volatile constituents.

Comparison Example 2

10.615 kg per hour of a graft polymer containing 14.8% of water areadded to a double-thread twin-shaft screw, the screw having aself-cleaning profile and the shafts rotating in the same direction.3.025 kg per hour of a styrene/acrylonitrile copolymer (SAN) with anL-value of 60 are melted in a side extruder and added at a melttemperature of 230° C. to the main extruder. The twin-shaft screw has ahousing diameter of 34 mm and an axial spacing of 28 mm. The free volumeof the screw is 1.03 1. The rotational speed of the screw shafts of themain extruder is 150 revolutions per minute. After the product streamshave been combined, the resulting product passes through a mixing zonecontaining kneading elements. The major proportion of the moisture isthen evaporated in a degassing zone that is operated at normal pressure.The residual degassing takes place in two further degassing zones thatare operated at 500 mbar and 20 mbar absolute pressure. The product isextruded at a temperature of 220° C. The mean residence time of theproduct in the twin-shaft screw is ca. 100. The resultant product iskneaded in a ratio of 40:60 with styrene/acrylonitrile copolymers (SAN)with an L-value of 60 and is extruded in the form of platelets for thecolour evaluation. As a comparison, a graft polymer that has undergone athermal drying is kneaded with a styrene/acrylonitrile copolymer (SAN)with an L-value of 60, so as to produce the same rubber content. Acomparison of the colour of the extrusions shows that the colour hasbeen adversely affected by the compounding of the moist product in thetwin-shaft screw.

What is claimed is:
 1. An elastomer-modified thermoplastic comprising amixture of a rubber, surrounded by a sheath of a thermoplastic materialA by graft polymerization, and a thermoplastic material B, and whereinthe rubber is produced by emulsion polymerization and wherein theelastomer-modified thermoplastic contains no more than 320 ppm ofresidual volatile components.
 2. An elastomer-modified thermoplastic asin claim 1 containing no more than 60 ppm of residual volatilecomponents.
 3. An elastomer-modified thermoplastic as in claim 1 in theform of a moulding.